1. Field of the Invention
This invention relates to an engine valve and a method for producing the engine valve used as an exhaust valve and an inlet valve of an automotive engine.
2. Description of the Prior Art
The exhaust valve and the inlet valve of the engine for automobiles are required to be excellent in corrosion resistance and oxidation resistance, to say nothing of high temperature strength, and further required to have excellent abrasion resistance at the positions to be in contact with a valve seat, a valve guide and a push rod.
Recently, demands become strong for increasing rotational speed and generating power of the engine and for improving efficiency and fuel comsumption of the automotive engine, and lightening and further improvement of the heat resistance are requested earnestly concerning the engine valve.
Heretofore, Ni-alloyed heat resisting steel and Ni-based heat resisting steel have been used as materials for the engine valve and a method of making the valve stem hollow in core has been introduced in order to reduce the weight of the engine valve, but it is not possible to say that the demand for lightening is satisfied sufficiently.
On the other side, Ti-based alloys are tried to be used as lightweight materials and put into practice in some engines. Although Ti-based alloys are applicable as materials for the inlet valve, it is not suitable to use as materials for the exhaust valve of high powered engine since Ti-based alloys are merely proof against high temperature up to about 500.degree. C.
Accordingly, in recent years, intermetallic compound materials, which are lightweight and excellent in the strength at high temperature, are tested to be used as material for valve motion-related members of the engine such as the engine valve.
Among them, application of Ti-Al based alloy including intermetallic compound is disclosed as materials for the valve in Japanese Patent disclosure Kokai) No. 61-229907/86 and No. 2-47278/90, for example. The engine valve made of Ti--Al based alloy of this kind has a shape as shown in FIG. 5, for example.
An engine valve 51 shown in FIG. 5 is made of Ti--Al based alloy and provided with a valve head 53 at an end of a valve stem 52, the velve head 53 has a head face 53a, a valve face 53b and head back 53c, and the valve stem 52 has a groove 54 and a tail end 55 at the opposite end thereof.
However, in the conventional engine valve 51 formed from Ti--Al based alloy, there is a problem in that the abrasion resistance of the engine valve 51 is unsatistactory at the tail end 55.